The atmosphere is composed of approximately 78% by volume nitrogen and 21% by volume oxygen, with other gases, principally argon, constituting less than 1% by volume. In most industrial processes requiring oxygen, air is used as the oxygen source. For certain industrial processes and in certain medical treatment procedures, there is a need to enrich the oxygen content of air. Where such needs exist, the air is enriched by being mixed with essentially pure oxygen obtained by low temperature fractionation of liquid air. The preparation of such oxygen-enriched airs is inherently expensive by reason of the large amount of energy required to liquify air. It is apparent that there is a long-term and continuing interest in the development of lower cost processes for providing oxygen/nitrogen gas mixtures having an oxygen content in excess of the 21 volume % level present in air.
It is known that both nitrogen and oxygen can pass through thin films of certain polymeric materials such as polyesters, nylons, polyethylene, diene rubbers such as natural rubber and polybutandiene, silicone rubbers, polyalkylene sulfones, and others. The rate at which a gas permeates or diffuses through a polymeric membrane is defined by Formula (1). ##EQU1## where: J is the gas flow rate (flux) through the membrane expressed in
(cm.sup.3 at STP).multidot.cm.sup.-2 .multidot.sec.sup.-1 .multidot.cm Hg.sup.-1. PA2 (cm.sup.3 at STP).multidot.cm.multidot.cm.sup.-2 .multidot.sec.sup.-1 .multidot.cm Hg.sup.-1.
P is the permeability constant of the membrane expressed in PA1 .DELTA.P is the differential pressure across the membrane expressed in cm Hg. PA1 A is the cross-sectional area of the membrane expressed in cm.sup.2. PA1 t is the membrane thickness expressed in cm.
The permeability constant, i.e., P in Formula (1), is a function both of the polymeric material from which the membrane is fabricated and the particular gas permeating the membrane. With all known polymeric membranes, the permeability constant for oxygen is higher than the corresponding permeability constant for nitrogen. The difference between the permeability constants for oxygen and nitrogen suggests that a oxygen/nitrogen gas mixture enriched in oxygen can be prepared by passing air through a polymeric membrane. The amount of oxygen enrichment possible is a function of the difference in the two permeability constants, which can be characterized as an ideal separation factor .alpha. which is defined by Formula (2). EQU .alpha.=P.sub.O.sbsb.2 /P.sub.N.sbsb.2 (2)
While the possibility of enriching the oxygen content of oxygen/nitrogen mixtures by passing air through a polymeric membrane is theoretically attractive, to date a number of factors have prevented any significant use of this technique. For such a system to be economically attractive, the membrane selected should have both a high permeability constant for oxygen and a large ideal separation factor. Permeability constants and ideal separation factors for oxygen and nitrogen obtained with a number of representative polymers are shown in Table I.
TABLE I ______________________________________ Polymer P.sub.O.sbsb.2.sup.(1) P.sub.N.sbsb.2.sup.(2) .alpha.O.sub.2 /N.sub.2 ______________________________________ Polyethylene 0.022 0.005 4.4 Terephthalate Nylon 6 0.038 0.010 3.8 Butyl Rubber 1.3 0.31 4.2 Polyethylene 3.0 1.3 2.3 Polystyrene 6.4 2.2 2.9 Natural Rubber 30 12 2.7 Silicone Rubber 600 260 2.3 ______________________________________ .sup.(1) P.sub.O.sbsb.2 .times. 10.sup.-10 .sup.(2) P.sub.N.sbsb.2 .times. 10.sup.-10
The above data indicate that, invariably, polymers having large ideal separation factors have low permeability constants for oxygen. Similarly, polymeric materials having large permeability constants for oxygen have relatively small separation factors. Thus, to obtain significant oxygen enrichment of oxygen/nitrogen mixtures, it is necessary to accept low oxygen permeability constants. To obtain significant gas flow across a membrane, it is necessary to accept low levels of oxygen enrichment.
To obtain significant flow rates through a membrane, the membranes employed must be very thin. Obviously, very thin membranes are difficult to fabricate and are quite fragile and subject to frequent breakage.
It is apparent that, if oxygen enrichment of oxygen/nitrogen gas mixtures is to be obtained on a practical basis, it is essential that the art develop thin gas permeable membranes which have reasonable mechanical strength, have reasonably high oxygen permeability coefficients, and have reasonably high O.sub.2 /N.sub.2 separation factors. It is the principal object of this invention to prepare such membranes.